Control grease pencil keyframes with a slider, like a shape key?
that is something you can do in Blender and it's the method we use.
Here's a quick tutorial,

Draw all your frames on a grease pencil object
In the modifiers tab add a time offset modifier and set it to Fixed Frame. The frame number will be the frame selected/shown on screen.
Use a driver to control which frame to show by right clicking the frame number and clicking add driver. Here you'll have to tweak settings dependent on your bone size and desired Active area. We generally use the distance operator ie, when the selector bone touches the smile bone it shows a smile.

Thanks
Matt